The influence of adenovirus transformation on expression of class I major histocompatibility antigens in the hamster is under investigation. Hamster embryo cells transformed with adenovirus 2, adenovirus 12, polyoma and SV40 are being examined, along with untransformed cells as a control. Results to date indicate that, at the RNA level, adenovirus 12 transformants have greatly reduced levels of mRNA encoding class I MHC antigens, a result in accord with findings for rat and mouse cells. However, cell surface expression of these antigens is not diminished to the same extent, so adenovirus 12 transformed cells express levels of class I antigens at the surface very nearly equivalent to those of most adenovirus 2-transformed cells or untransformed cells. A heterogeneity of class I antigen levels in the adenovirus 2 transformants has also been found. Continuing study of hamster female protein (FP) has focused on some aspects of the protein structure which may account for some of the unusual properties of FP in serum. The nature of its carbohydrate moiety has been defined and the arrangement of its sulfhydryl groups and disulfide bonds is under study. Free sulfhydryl groups have been found but their source is as yet known. Rabbit immunoglobulin latent allotypes have been further defined by protein sequence.